Like Archeryrob mentioned, movement is one of the larger problems. I've watched new hunters from a distance. One of the bigger mistakes I've seen is they try to look every which direction. Thier body may be still, but their head moves around.

Imagine a head size piece of paper, brown on one side, white on the other, on a stick. Imagine you are a hundred yards off and someone is slowly spinning that piece of paper on the stick. You can pick it up quick. Now imagine that same piece of paper moving really slow. Or imagine it moving with the wind and the sway of the branches.

Most likely the Deer know you are around someplace, they are smelling parts per million. Not enough to really spook them, but enough to make them watchful and nervous. Then throw in what I call a flash, your white face jerking around. If they aren't out of there, they sure as heck aren't going to get much closer to the spot they saw that unnatural movement. I'd rather miss an occasional Deer than blow my spot by trying to look everywhere at once.

Throw in a unnatural sound, like the thump of your rifle on a rest or foot stamp from shifting your feet and that Deer is out of there.

The really spooky Deer are going to move off or bolt when they get the slightest whiff of human. Some are a bit more tolerant. If they get a nose full under fifty yards, shifting breeze or walking through your scent cone, most times they are going to bolt, sometimes they will hide then slink off.

In a normal day you sweat a pint, you breath a pint and urinate 3-4 pints. You can mitigate your scent some. But seriously, studies have shown Deer can smell stuff on a molecular level from astounding distances.

Also something to keep in mind, even on a still day air rises as it warms up in the morning and spreads your scent wider.

Like Alleyyooper mentioned, I've seen the Bucks let the Doe run interference for them. Especially the older more experienced Bucks, the survivors.

I go out in the off season and hunt predators. I watch the Deer.